How the Great Pluto War became a quagmire

Before the dust even settled after the Great Pluto War at the International Astronomical Union's General Assembly in Prague, one thing became clear: There will never be an accepted scientific definition for the term "planet."

Rather than crafting an acceptable definition, the IAU alienated members, put the group's authority in jeopardy and fueled schisms among astronomers on theoretical grounds and even nationality.

And the whole affair was scientifically pointless, many astronomers say.

The controversial planet-definition resolution, passed Aug. 24 in a vote by just 424 IAU members, will not stand as worded. About 300 astronomers have pledged not to use it, and many others say it must be redone to eliminate contradictions. It will be reworked, at the least, and possibly overturned at the 2009 IAU General Assembly in Rio de Janeiro, Brazil.

Meanwhile, the debate — which the IAU limited to defining round things in our solar system — was a neighborhood nomenclature brawl amid a universal war of words. Any terminology that might be relevant to our little solar system will be laughably inadequate if applied across the galaxy.

Shortly after the Prague vote, I posed a series of questions about the new definition's merits and shortcomings to several astronomers, among them Geoff Marcy at the University of California at Berkeley. Marcy and his colleagues have found more planets beyond our solar system than any other team.

"Your questions imply that a definition of the word 'planet' is useful scientifically. That is a view not shared by many professional planetary scientists," Marcy replied. "The astrophysics of planetary bodies is so rich and complex that defining 'planet' has never been an issue under discussion among professionals," he said. "The taxonomy of asteroids, comets, moons, planets and brown dwarfs is far too limited to capture the diversity of their origins and internal constitutions."

Diverse indeed. During 2006, the tally of known extrasolar planets surpassed 200, and the range of sizes and setups illustrates why a universal definition is impossible in light of the fact that scientists are sharply divided on what to call Pluto.

Arguing since 1990
The debate over what constitutes a planet flared up after the 1990 discovery of the first round objects orbiting another star. The three so-called pulsar planets are about the same size as Earth. They are often forgotten in discussions about exoplanets. Some astronomers don't see them as planets at all, because they orbit a fast-spinning, dead star that cannot support life.

Slideshow   Month in Space Catch a blast from the sun, a clash between galaxies and other outer-space highlights from October. more photos

Other worlds several times the mass of Jupiter float freely in space; they have no host star. Are they planets? Other oddities abound.

"It is a little-known fact that nearly 25 percent of the known extrasolar planets are in binary- or multiple-star systems," said Stephen Kortenkamp, a research associate at the University of Maryland. "That further complicates the notion of creating a universal definition of planet."

One day during what Kortenkamp calls the Great Pluto War, he browsed his dictionary. "I see lots of words that have multiple definitions, depending on the context in which they are used," he told me back then. "I don't see why the word 'planet' can't be treated the same way."

Kortenkamp figures "planet" means one thing in our solar system and something else around other stars, and also has varying meanings for geologists or planet-hunters or the public. "The IAU would have been better off with this approach rather than trying to dictate a single definition for what is really a cultural term that means different things to different groups of people," he said.

MORE FROM SPACE

Space Section Front

After letdown, solar-sail project rises again Russian room ready for space station launch Black holes: Powerhouses of the universe Space elevator team wins $900,000 from NASA Space junk buzzes station while crew sleeps Moon’s ‘friends’ say no to future lunar crashes New survey finds 22 of the earliest galaxies Cosmic Log: See starbirth up close Stars: Cosmic road signs to intelligent aliens?   Month in space: What a blast! Space Section Front   Add Space headlines to your news reader: